I voted 26 million. I think that is optimistic and my highest prediction but I think it has a shot at it. Odyssey will be 19.50 or so this year, and I could see it doing 2 million a year or just under for the next three years, and then getting that last half million after Switch is retired.
I think there will be one more 3D Mario on Switch, though I suppose it is possible they won't do another new 3D Mario if they are bringing an 3D All-Stars type collection this Fall, plus 3D World port at some point. Cuz that would already be 3 separate 3D Mario games, and they might figure that is enough for one system, so maybe the next new one could just be a Switch 2 launch title. Whenever the next new one hits, I think it'll either be Galaxy 3 or Odyssey 2. Galaxy is much loved and fans would go crazy over one final Galaxy game, but Odyssey is the best selling 3D Mario by a long shot and I'm sure they could do even more with the cappy mechanic, so either one, or possibly a Galaxy game with cappy would be great, before moving on to something totally new.
I wouldn't mind them shifting resources to other series instead of another 3D Mario on Switch if Switch ends up with Odyssey, NSMBUD port, 3D Mario Collection, 3DWorld port, and a fresh 2D Mario game, as that's already a ton of core Mario games.